1. Field of the Invention
The present invention relates to a gamma-correction circuit for a blue color video signal in a television, and more particularly to a gamma-correction circuit for a blue color video signal which can prevent the deterioration of focusing in a projection television by limiting influx of the peak-current value of the gamma-corrected blue color video signal into a blue color cathode ray tube (CRT).
2. Description of the Prior Art
In general, a projection television has separate CRTs for red, green and blue color signals to display a picture on a screen. As shown in FIG. 1, the blue color video signal Vin applied to the corresponding blue color is displayed as being distorted due to the characteristic of blue color CRT, unlike the red or green. For this reason, a gamma correction circuit is provided on the blue color CRT to gamma-correct such distortion of the video signal.
FIG. 2 shows a conventional gamma-correction circuit for the blue color video signal Vin in a projection television. The circuit includes a gamma-correction section 10 for gamma-correcting the blue color video signal Vin inputted from an image processing section (not illustrated in the drawing) of a projection television; a video amplifying section 20 for amplifying the blue color video signal Vin, which has been gamma-corrected by the gamma-correction section 10, to a predetermined level; and a blue color CRT 30 for displaying the blue color video signal Vin outputted from the video amplifying section 20.
The gamma-correction circuit for a blue color video signal in a projection television as constituted above operates as follows.
If a blue color video signal Vin is inputted to the gamma-correction section 10 from the image processing section of a projection television, the gamma-correction section 10 gamma-corrects the inputted blue color video signal Vin. The blue color video signal Vin is inputted to the amplifying section 20 after being gamma-corrected in an exponential function, as illustrated in FIG. 3. The gamma-correction is performed by the gamma-correction section 10 in a manner such that the blue color video signal Vin is amplified to have opposite characteristics to that of the anode current versus brightness of the blue color CRT 30 for the blue color video signal Vin. Accordingly, the anode current versus brightness of the gamma-corrected video signal has similar characteristics to that of the red or green color video signal.
Thereafter, the video amplifying section 20 amplifies the gamma-corrected video signal to a predetermined level, and then inputs the amplified blue color video signal Vin to the blue color CRT 30. The Blue door CRT 30 displays the image on a screen (not illustrated in the drawing) via a projection lens (not illustrated in the drawing) and a light path conversion mirror (not illustrated in the drawing). Accordingly, if the blue color video signal Vin is gamma-corrected by the gamma-correction circuit, the characteristics of the white balance becomes less dependent on variation of the anode current. Ultimately, images of high quality can be displayed on the screen.
However, the conventional gamma-correction circuit in a projection television as constituted above discloses spiral characteristics in the amplifier of the gamma-correction section 10 used for gamma-correction. Therefore, the gamma-corrected video signal is inputted to the Blue color CRT 30 as peak-beam current which has much greater brightness than that graphed as (B) in FIG. 4 in comparison with the beam current prior to undergoing the gamma-correction illustrated as (A) in FIG. 4. Accordingly, as the peak-beam current flows into the blue color CRT 30, the characteristic of focusing deteriorates, thereby lowering the picture quality.